Abstract: This paper presents how to recognize targets under complicated background of cloud in sky by stimulating optical correlation. Some factors influencing recognition are calculated, such as changes of background intensity, edge detection of input image or matched filter and so on. Simulation results show that targets can be found in weaker background by direct optical correlation of input image and its matched filter without edge detection. When background becomes stronger, edge detection of input image is the key to improve target recognition capacity, but it weakens the intensity of correlative peak.